Developing a good collection of GPL'ed textures would be really great. I've been working on a new set of textures for some time now, and I'd be glad to release them under the GPL if I can get my photo reference sources to agree to it.
I think the huge lack of good GPL'ed textures out there is really holding this project back a lot, especially for mappers. A project where a few good texture artists collaborate to release even one or two coherent high-quality texture packs under the GPL would go a long way to making this game better.
Edit: My reference sources agreed, so you can look forward to a texture set from me sometime in the next month or two
You can see a preview of them here